Recently, a liquid crystal display device having a liquid crystal panel and a backlight source has come into wide use as a TV set, display device, etc. The liquid crystal display device poses a problem of motion blur occurring in displaying moving images when operated by a hold-type drive method of constantly turning on the backlight source.
FIG. 7(a) depicts a visually recognized image F2 that is recognized by a viewer when an original image F1 moves on a liquid crystal display panel driven by a hold-type drive method of constantly turning on a backlight source. As depicted in FIG. 7(a), in a state of hold-type drive, motion blur due to an afterimage persisting on the human retina is created on the visually recognized image F2 that is recognized by the viewer when the original image F1 moves.
A conventional method is known as a method of preventing motion blur occurring in displaying moving images by intermittently turning on a backlight source to carry out pseudo impulse drive (see, e.g., patent document 1.)
Particularly, as disclosed in patent document 1, a backlight scanning may be executed in such a way that a plurality of light sources corresponding to a plurality of display areas in a vertical direction of a liquid crystal panel are intermittently turned on in sequence in synchronization with image writing in the display areas.
For example, a backlight source 31 depicted in FIG. 2 includes rows of LED light source groups L1 to L12 corresponding to a plurality of display areas in the vertical direction of a liquid crystal panel. Each of the LED light source groups L1 to L12 has a plurality of horizontally arranged LED light sources 31a. 
FIG. 8 depicts an example of the result of execution of the backlight scanning by the backlight source 31. As shown in FIG. 8, in the backlight scanning, the LED light source groups L1 to L12 are intermittently turned on in sequence to realize pseudo impulse drive.
More specifically, in the backlight scanning, when a vertical synchronization signal (FIG. 8(a)) is received, the first horizontal synchronization signal is then received, and a time for writing an image signal in the first line arrives (FIG. 8(b)), the light source group L1 is turned off for about 8.3 ms and then is turned on for about 8.3 ms (FIG. 8(c)). When writing an image in an area corresponding to the LED light source L1 is finished and a time for writing an image in an area corresponding to the LED light source L2 arrives, the light source group L2 is turned off for about 8.3 ms and then is turned on for about 8.3 ms (FIG. 8(c)). In the same manner, other LED light source groups L3 to L12 are each turned off for about 8.3 ms and then are turned on for about 8.3 ms (FIG. 8(c)) when a time for writing an image in each of areas corresponding to the LED light source groups L3 to L12 arrives.
FIG. 7(b) depicts a visually recognized image F3 that is recognized by the viewer when the original image F1 moves on the liquid crystal display panel as the backlight scanning is executed. As shown in FIG. 7(b), the backlight scanning reduces the motion blur of the visually recognized image F3 recognized by the viewer to allow less motion blur than in a case of the hold-type drive (FIG. 7(a)).
A technology called multi-speed response liquid crystal technology has become widely known in recent years, according to which the frame frequency (60 Hz) of TV broadcasting video is increased up to an n-fold frequency (120 Hz, 240 Hz, etc.) to enable multi-speed response display. This technology shortens a display time of 1 frame, thus reduces image persistence feelings.
In this technology, as disclosed in patent document 2, a method of generating an interpolation image based on two or more consecutive frame images and inserting the interpolation image between frames to be displayed (equivalent to a first multi-speed drive mode, which is hereinafter referred to as “interpolation image insertion mode”) and a method of displaying the same frame image n times in a row (equivalent to a second multi-speed drive mode, which is hereinafter referred to as “overlapped image output mode”) are used to increase the frame frequency to an n-fold frequency.
To increase the frame frequency to an n-fold frequency, the frequencies of a vertical synchronization signal and a horizontal synchronization signal are increased to n-fold frequencies. As depicted in FIG. 9, for example, in the overlapped image output mode, a series of the backlight scanning are executed to display each of frame images A, A, B, and B, during which the execution cycle of the intermittent turning-on action in the backlight scanning become a twofold cycle.
FIG. 7(c) depicts a visually recognized image F4 that is recognized by the viewer when the original image F1 moves on the liquid crystal display panel as the interpolation image insertion mode is executed. FIG. 7(d) depicts a visually recognized image F5 that is recognized by the viewer when the original image F1 moves on the liquid crystal display panel as the overlapped image output mode is executed.
As shown in FIG. 7(c), in the interpolation image insertion mode, an interpolation image based on preceding and following frame images is inserted between frames. As a result, smooth moving image display is realized and no deteriorated form of a multi-contour due to multi-speed drive in the backlight scanning is observed.